1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a manufacturing method therefor, and more particularly to an improvement in the number of rewriting information in a split-gate-type flash memory.
2. Description of the Relatrd Art
A method of manufacturing a split-gate-type flash memory, which is a conventional nonvolatile semiconductor memory device (refer to, for example, Japanese Patent Application No. 9-42478) will now be described with reference to the drawings.
As shown in FIG. 37, the foregoing split-gate-type flash memory is a flash memory incorporating a control gate 100 formed from an upper portion of the floating gate 102 to the side portion of the same through an insulating film 101. Referring to FIGS. 34 to 37, the processes for manufacturing the foregoing flash memory will now be described.
Initially, a first gate insulating film 104 in the form of a SiO.sub.2 film is formed on a semiconductor substrate 103. Then, a polycrystalline silicon film 105 is laminated. Then, P ions are implanted to form the floating gate 102 shown in FIG. 37 and attempted to be formed (see FIG. 34).
Then, a silicon nitride film 106, which is an oxidation resistance film, is formed in such a manner that the polycrystalline silicon film 105, which is a region in which the floating gate 102 will be formed, is exposed. The silicon nitride film 106 is used as a mask to form a mini LOCOS oxide film 107 (see FIG. 35).
Then, the silicon nitride film is etched, and then the mini LOCOS oxide film 107 is used as a mask to etch and remove the polycrystalline silicon film 105 so that the floating gate 102 is formed.
Then, the first gate insulating film 104 is isotropically etched with hydrofluoric acid etching solution so that the first gate insulating film 104 is left in only a portion directly below the floating gate (or etched and removed in such a manner that the first gate insulating film 104 is somewhat left around the floating gate 102).
Then, a second insulating film 101 made of silicon oxide (a thermal oxidation film or a CVD film) is formed on the entire surface (see FIG. 36).
Then, a polycrystalline silicon film is formed on the second insulating film 101, and then a patterning process is performed so that a control gate 100 extending from the upper portion of the floating gate 102 to the side portion of the same is formed. The thus-formed floating gate 102 and the control gate 100 are used as masks when impurities are implanted onto the semiconductor substrate 103. Thus, a source region and a drain region (not shown) are formed. As a result, the split-gate-type flash memory can be formed.
A program has been written on the above-mentioned split-gate-type flash memory by turning transistors of memory cells (hereinafter called "selected cells"), on which data must be written, on to implant electrons into the floating gate 102.
Moreover, as a region surrounded by a dashed line shown in FIG. 37, the polycrystalline silicon film 105 on the upper surface of the floating gate 102 is oxidized so that a mini LOCOS oxide film 107 is formed on the polycrystalline silicon film 105. Then, a projection 108 is formed at the leading end of the bird's beak. Concentration of electric fields to the projection 108 is used to remove electrons of the floating gate 102 from the floating gate 102 toward the control gate 100 so that the program is erased.
FIG. 5 is a plan view showing the above-mentioned split-gate-type flash memory. FIG. 37 is a cross sectional view taken along line B--B. A rectangular portion (hatched with dots) indicated with an elongated alternate long and short dash line is in the form of a LOCOS oxide film, while rectangular portions indicated with an alternate long and two short dashes line and a dashed line are the floating gate 102 and the mini LOCOS oxide film 107. A solid line extending laterally indicates the control gate 100.
The above-mentioned projection 108 is formed mainly by dint of the shape of the bird's beak of the mini LOCOS oxide film. That is, the bottom surface of the bird's beak has an inclination which is raised in a direction toward the periphery. Therefore, the sharp projection 108 is formed on the entire circumference of the floating gate.
However, the above-mentioned inclination of the mini LOCOS oxide film 107 is unsatisfactory in a viewpoint of the shape of the projection.